Galileo formulated the principle of relativity in order to show that one cannot determine whether the earth revolves about the sun or the sun revolves about the earth. The principle of relativity states that there is no physical way to differentiate between a body moving at a constant speed and an immobile body. It is of course possible to determine that one body is moving relative to the other, but it is impossible to determine which of them is moving and which is immobile. The laws of physics are identical in systems moving one at a constant velocity in relation to the other, so that one cannot conduct any physical experiment capable of indicating if the body is immobile or in motion.
In fact, there is no meaning in the concept of a moving body, without reference to its movement relative to another body. Velocity is only relative velocity.
As to the question whether the earth revolves about the sun or the sun revolves about the earth, Galileo explained the significance of the movement by means of the example of a ship. This principle is closely associated with the law of inertia.
At the beginning of the 20th century, the principle of relativity was accorded new importance in the theories of relativity proposed by Einstein and Poincare. Based on the principle which states that the laws of physics are identical in a moving or static system, a new mechanics was developed which rejected Newtonian mechanics and led to changes in the conception of space and time.
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